Good editors work inside the structures of thought
When you understand the structures that we weave thought into — the structures that make up phrases, clauses, entire sentences — when you learn how to shape and reshape them for different purposes, then you command thought, you command language.
For any sentence you construct, you should know why those words, why that arrangement, why this particular sentence in this particular place.
Writers sometimes do this intuitively — or “intuitively” after much study and long practice. And yet, for all this training, writers often work by feel, knowing where they want to go with a particular sentence or passage, but not necessarily why.
Ah, but the why is so important.
Editors, good editors, understand the why. Grammar is not (as Joan Didion put it) the piano they play by ear. Nor is syntax or usage or rhetoric or lyricism, or any of the tools and techniques of writing. No, editors understand the how and the why. They feel the rhythms, the harmony, the sweet strains of thought intertwining. But they also understand the composition. They can write the music.